1. Field of the Invention
The present invention relates to systems for determining wall thicknesses along a length of closed pipe such as a pipeline in an industrial installation; more particularly, to such a system using photographic images of x-ray exposures to infer such wall thicknesses; and most particularly, to such a system wherein the film and the x-ray source are sequentially and accurately positioned at a predetermined array of axial and radial locations to provide a three-dimensional assessment of pipe wall thickness over a predetermined length of pipe.
2. Discussion of the Related Art
Pipelines in many industrial applications are subject to internal corrosion over time. Such pipelines may carry hazardous materials and may be operated at elevated temperatures and/or pressures. Thus, the thickness of the pipe wall can be a critical safety consideration, and accurate determination of remaining wall thickness can be an important part of a maintenance and safety program.
In general, such pipelines are closed and filled with the working materials; thus, it is impractical to make direct physical inspections and measurements of wall thickness. Various inferential techniques have been applied. One commonly employed technology involves ultrasonic measurement, but this typically requires that any pipe covering such as insulation be removed and a smooth surface be ground oh the pipe exterior for coupling of an ultrasonic horn, and further, that the pipe be emptied. Thus, ultrasonics generally cannot be applied to a pipeline in service.
Numerous systems involving radiography have been proposed. Such systems generally have a radiation source, typically gamma rays or x-rays, disposed adjacent a front wall portion of a pipe; and sensing means, such as photographic film or electronic receptors, disposed adjacent the back wall portion of the pipe, opposite the front wall portion. For purposes of the following discussion, the pipe, although typically cylindrical, may be thought of as having left and right side wall portions, respectively, for a vertical pipe, corresponding to upper and lower side wall portions for a horizontal pipe, connecting the front and back wall portions, although the limits of any wall portion of a cylindrical form cannot be strictly defined.
U.S. Pat. No. 4,692,936, issued Sep. 8, 1987 to Billeaudeaux, discloses a radiation source off-spaced from the front wall of a pipe and a flat photographic film disposed behind the pipe. The apparatus further includes a comparative scale body constructed of brass and having a range of known thicknesses. The step-scale body is radiographed adjacent to and simultaneously with the pipe portion of interest, and the thickness of the pipe wall may be inferred from the radiograph by comparing the wall image with the scale body image.
U.S. Pat. No. 4,974,246, issued Nov. 27, 1990 to Heiskel, discloses a radiation source and film in the conventional relationship except that they are separated axially along the pipe such that the centerline of the radiation forms an incident angle with the pipe of between 30xc2x0 and 75xc2x0. This system is said to be useful in checking for internal pipe corrosion at hangers or supports where orthogonal radiography is impractical.
U.S. Pat. No. 5,614,720, issued Mar. 25, 1997 to Morgan et al., discloses an apparatus having a scannable radiation source off-spaced from the front wall of the pipe and a semicircular array of radiation receptors disposed around the back half of the pipe. The electronic receptors provide an array of signals which are collected and analyzed by a computer alogrithm to determine deviations from the expected thickness of the pipe at a variety of radial angles. The apparatus may be moved around the pipe as well as along the pipe to make multiple determinations which can be combined by the algorithm to provide a model of the wall thickness both radially and axially of the pipe.
U.S. Pat. No. 5,864,601,. issued Jan. 26, 1999 to Cattorini et al., discloses a conventional arrangement between radiation source and photographic film. Two shims of different thickness are placed adjacent an area of suspected corrosion in the back wall of the pipe and are radiographed therewith.
Thickness of the corroded area is inferred by computer analysis of optical density data from the shim and pipe back wall images.
In general, the prior art approaches use optical density analysis to infer thickness of portions of the back wall of a pipe. of the just-discussed patents, only U.S. Pat. No. 4,692,936 discloses apparatus and method for determining pipe wall thickness by measuring directly the radiation shadow of the side wall, as shown in that patent""s FIGS. 3 and 4. But this disclosure makes no provision. for sytematically examining closely-spaced portions of a pipe, both radially and axially, and for repeatedly making such examination with-a high degree of accuracy to monitor the progress of corrosion therein over time.
What is needed is a simple yet reliable system for measuring optically, by radiography and subsequent trigonometry, both apparent and true wall thickness of a pipe at a predetermined array of radial and axial positions, and for being able to repeat such measurements at each of such positions at any desired intervals of time, wherein the pipe may be under operating conditions at the time of measurement.
It is a principal object of the invention to provide an improved system for radiographically determining the wall thickness of a pipe at any desired location.
It is a further object of the invention to provide an improved system for radiographically determining the wall thickness of a pipe at any desired location relative to other wall thicknesses at other radial and/or axial pipe locations.
It is a still further object of the invention to provide an improved system for monitoring the progress over time of wall thinning of a pipe in service at any desired radial or axial location along the pipe.
Briefly described, a system in accordance with the invention includes a supporting framework removably attachable to the outer surface of a pipe or pipe covering and supportive of a holder for photographic film for recording x-radiation from an x-ray source off-spaced from the holder. The holder and the source are disposed on opposite sides of the pipe in a first plane which, includes the axis of the pipe. The source is axially movable so that successive exposures may be made of the pipe along the first axial plane on a single piece of film or successive films. Each such image preferably includes both the upper and lower side walls of the pipe. The film holder is also radially indexable to the pipe via a plurality of regularly-spaced radial bores in arcuate end pieces of the supporting framework, so that the film holder and source may be disposed successively in a plurality of other axial planes radially displaced from the first axial plane.
In each such plane, a fresh piece of film is installed in the holder. After processing of all films, the pipe side wall thicknesses may be determined for each axial and radial location by measurement of the film images, thereby providing a substantially three-dimensional display of wall thickness of the pipe over the axial length of the film exposures made. Thickness data from the radiographs may be entered readily into any of various well known commercial software programs for computer analysis if so desired. At each axial position of the supporting framework, the exact locations of the clamping feet may be inscribed on the pipe or pipe covering, such that later determinations may be made from exactly the same locations, thus permitting precise monitoring of pipe wall thickness over time.